Monochromators

A monochromator is an optical instrument that transmits a narrow band of wavelengths selected from a broad light source. It is used to isolate a single wavelength or a small spectral range for experiments in spectroscopy, photometry, calibration, and various optical analyses.

Operation relies on a dispersive element such as a diffraction grating or a prism to spatially separate light into its component wavelengths. Light from the source enters through an entrance slit, is dispersed by the element, and then passes through an exit slit to define the transmitted bandwidth before reaching a detector. By rotating the dispersive element or adjusting slit widths, a particular wavelength and its bandwidth are selected. The spectral resolution, often expressed as R = lambda/delta lambda, depends on wavelength, slit width, dispersion, and optical quality.

Two common configurations are prism-based and grating-based monochromators. Grating monochromators—frequently arranged in Czerny–Turner or Rowland-circle geometries—offer higher resolution and broader spectral coverage. Prism monochromators rely on refraction for dispersion and are typically used when very high resolution is not required or when transmission in a specific region is favorable. Modern designs optimize throughput and stray-light rejection.

Applications include ultraviolet–visible and infrared spectroscopy, quantitative photometry, calibration of light sources, and selective optical excitation in fluorescence and Raman experiments. Scanning monochromators vary the output wavelength during measurement, while fixed-bandpass configurations use detectors with synchronized spectral filtering.